Dry strength paper containing unbleached cellulosic fibers and a non-thermosetting cationic polyamine



United States Patent 3,258,393 DRY STRENGTH PAPER CONTAINING UN- BLEACHED CELLULOSIC FIBERS AND A NQN-THERMOSETTING CATIONIC POLY- AMINE Norman Thorndike Woodberry and Walter Florus Reynolds, Stamford, Conn., assignors to American Cyanamid Company, Stamford, Conn., a corporation of Maine No Drawing. Filed Jan. 30, 1964, Ser. No. 341,413

Claims. (Cl. 162-164) The present invention relates to paper of low wet tensile strength but of substantially improved dry tensile strength resulting from an adsorbed content of a cationic two-dimensional low molecular weight polymeric polyamine. The invention includes the paper itself and processes for the manufacture of the paper.

It has long been known that paper possesses both wet and dry tensile strength when the fibers are bonded together by an adsorbed content of a thermosetting cationic resin in thermoset or cured state, cf. Maxwell et al. US. Patent No. 2,559,220.

Paper of that type is made by forming an aqueous suspension of cellulose paper-making fibers, adding an aqueous solution of a cationic thermosetting amidogenformaldehyde resin to the suspension, sheeting the suspension to form a wet web, and drying the web at an elevated temperature. When applied to the fibers such resin is in low molecular weight water-soluble state. When the web is dried, the resin cross-links with itself and cures to macromolecules which are three-dimensional and which are consequently insoluble and infusible. Because of their size, the macromolecules bind the fibers together, and because the macromolecules are water-insoluble, the bond is water-resistant, so that the paper possesses wet strength.

Wet strength is a property not always desired in paper and a demand has therefore arisen for a paper which possesses substantially improved dry tensile strength while possessing substantially no wet tensile strength at all. In the past, this demand has been met by paper having an adsorbed content of an essentially linear or one-dimensional polymer, for example, a linear vinyl polymer. Such polymers are usually costly and of high molecular weight; cf. Wilson et al. US. Patent No. 2,884,057; Schuller et al. US. Patent No. 2,884,058; Woodberry et al. US. Patent No. 2,890,978; Woodberry US. Patent No. 2,959,514; and Padbury et al. US. Patent No. 2,963,396. The polymers disclosed in these patents produce their strengthening effect primarily by an entwining or entangling action, and do not form an enveloping substantially continuous cocoon-like network on the fibers such as is formed by the thermosetting polymers described above.

The discovery has now been made that paper possesses very substantially improved dry tensile strength (while possessing at most negligible wet tensile strength) when the fibers of which it is composed are bonded together by an ionically adsorbed content of a normally water-soluble cationic polymer which is two-dimensional as to its structure, which is non-thermosetting as to its chemistry, and which has a low molecular weight, i.e.,

a molecular weight in the range of about 1,000 to about 10,000.

In preferred instances the paper of the present invention has a dry tensile strength which is 25% in excess of the dry strength of paper which contains no polymer, yet such paper has a wet strength so low that the paper can be pulped by slushing in an ordinary beater, without use of steam jets or special chemicals.

The paper of the present invention is composed of interfelted anionic (i.e., acidic) cellulose fibers bonded together by the bridging action of one or more cationic polymers of the type herein described. These polymers are comparatively small and non-thermosetting in character. Necessarily, then, the bridging action of the polymers is local, and a cocoon-like integument does not form.

The paper may be composed of any of the common unbleached paper-making cellulose fibers, as these fibers are significantly acidic and hence have significant ionic adsorptive capacity for the cationic polymer therein. The paper possesses best dry tensile strength per unit weight of cationic polymer therein when it is composed of fibers carrying a high proportion of acidic substituents, particularly unbleached kraft fibers. Such fibers contain more than about 1% of their dry weight of acidic lignin material (believed to be chiefly lignin and lignin phenols) and hence possess a particularly large number of sites on which the polymer can be adsorbed. Fibers containing lignin sulfonic acid (for example unbleached sulfite fibers) are similarly useful.

The bonds between the fibers of the paper of the present invention are of the cellulose-lignin-polymerlignin-cellulose type; the polymer forms ionic bridges between the acidic loci of the fibers. These bridges are strong when dry, but, being ionic, they are weak when wet. This accounts for the ease with which the paper of the present invention can be pulped, and for its lack of wet strength.

The paper of the present invention is made by forming an aqueous suspension of unbleached beaten cellu lose fibers, adding thereto an aqueous solution of a cationic, two-dimensional non-thermosetting low molecular weight polyamine, forming the suspension into a water-laid web, and drying the web.

The pH at which the cationic polymers described herein are best adsorbed varies from instance to instance and depends principally on the number and type of electro-negative sites (cellulosic -OH and COOH groups and acidic lignin substituents) in the fibers, the extent to which these sites have been neutralized by polyvalent metal ions generally present in paper-making systems such as calcium and aluminum ions, and the charge density of the cationic polymer. Accordingly, in each instance the optimum pH is most readily found by laboratory trial. As a rule of thumb we have generally found the optimum pH to be Within the pH range of 5 to 7 or 8.

The amount of cationic two-dimensional polymer which is added in each instance should be at least s-ufiicient to produce an economic increase in dry tensile strength, and about 0.2% is about the least amount of polymer which justifies its use. Larger amounts of the polymer cause greater improvements in dry strength, and the improvement continues ro-ughly linearly until the amount of polymer added is sufiicient to neutralize substantially all of the lignin material present in the fibers. This amount is therefore preferred. Larger amounts can be profitably added up to the adsorptive capacity of the pulp, but the strengthening effect of each added increment becomes progressively less.

The polymers may be added at any convenient point in the paper-making system, at or upstream from the fan pump. They are rapidly adsorbed by the fibers.

The temperature at which the web is dried and the duration of the drying are not critical. The polymer, being substantially non-thermosetting, need not be subjected to a temperature in the normal paper drying-range. The invention contemplates, however, that the paper will be produced by drying on steam heated rolls in the range of 190 F.250 F. as is customary.

The process of the present invention tolerates the addition of a number of materials to the fibrous suspension. Thus, there may be added pigmentary material (e.g. ultramarine, clay, Ti and calcium carbonate), and basic dyes.

The paper of the present invention in finished form or as broke is easily pulped and reformed into paper, for example, by slushing the paper in approximately neutral (pH 5-8) water for a few minutes and then subjecting the stock to customary beating. An intensive beating is not necessary, and steam jets and oxidizing or reducing chemioals have not been found necessary.

A variety of normally water-soluble nonthermosetting two-dimensional cationic polymers are known which produce dry strength without producing wet strength when present in paper. In general, these polymers are prepared by joining together polyamine molecules by the use of a suitable cross-linking agent. The cross-linking agent is caused to undergo substantially complete reaction during preparation of the polymer, as a result of which the polymer is substantially non-thermosetting.

The amount of cross-linking agent used in the preparation of the polymer is small in comparison with the total functionality of the amine or polyamine used as primary starting material. At least sufficient cross-linking agent is used so that a polymer is formed which has a molecular weight of about 1,000; this is about the lowest molecular weight which produces a polymer of adequate dimensions for paper strengthening purposes. Such polymer, being essentially two-dimensional, is essentially fiat when adsorbed on a fiber surface.

On the other hand, the amount of cross-linking agent used is insufiicient, so that when the cross-linking agent has substantially completely reacted, the molecular weight of the polymer is not substantially in excess of about 10,- 000. We have found that two-dimensional polymers of greater molecular weight tend to impart a greater, and for many purposes disadvantageous, amount of wet strength.

The proportion of the starting amine or polyamine to the cross-linking agent is maintained sufiiciently high so that the number of amino nitrogen atoms in the final polymer product is suificient to render the polymer cationically substantive to the cellulose fibers. Our evidence indicated that for this purpose .the polymer should contain at least about 5 or nitrogen atoms on the average; some or all of these nitrogen atoms may be in quaternary ammonium state.

One polymer suitable for use in the present invention is formed by reacting epichlorohydrin or other linking agent with a mixture of water-soluble bifunctional and po-lyfunctional amines. The bifunctional amine is present in major molar proportion as major component of the amine mixture, and reacts with the epichlorohydrin to form a linear polyalkylenepolyamine. The polyfunctional amine is present in minor molar proportion and provides secondary amino linkages within the linear polyalklenepolyamine, which act as sites at which two-dimensional branching can and does take place. The molar ratio of the functionality of the bifunctional and polyfunctional amines used for the preparation of this polymer is about 1:1, so that neither the epichlorohydrin nor mixture of amines is present in significant excess.

The reaction is carried to substantial completion so that the polymer is non-thermosetting.

Suitable monomeric bifunctional amines for use in the aforementioned process include methylamine, ethylamine, ethanolamine, N,N'-dimethylethylenediamine, aniline, piper-azine, and ethyl a'minoacetate (ethyl glycinate).

Suitable monomeric polyfunctional amines for use in the aforementioned process include ethylenediamine, N-methylethylenediamine, ammonia, hydrazine, p-phenylenediamine, 3,3-irninsobispropylamine, diethylenetriamine, tetraethylenepentamine, and the corresponding polypropylenepolyamines.

In place of epichlorohydrin, glyoxal, 1,2-dichloroethane, diglycidyl ether, methylenebisacrylamide, 1,4-di-chlorobutene, tetrahydroaacroyltriazine, and adipoyl chloride can be used as the linking agents.

The polymer may and advantageously does contain inert groups as diluents. Such polymers can be prepared by reacting epichlorohydrin with the watersoluble product formed by condensing at amidation temperature a watersoluble dib-asic acid and a Water-soluble polyalkylenepolyamine; the amount of epichlorohydrin is about (or to /5 mol) per mol of combined dibasic acid present. The epichlorohydrin reaction is carried to substantial completion so as to form a non-thermosetting polymer. It is generally advantageous when the amino nitrogen atoms of the polymer are separated by between about 10 and 20 ionically inert atoms.

A polymer which contains ether linkages or spacers and which also gives good results can be prepared in similar manner by replacing part or all of the glycol with diethylene glycol.

The invention will be more particularly illustrated by the examples which follow. These examples constitute specific embodiments of the invention and are not to be construed as limitations thereon.

The polymers described below are normally watersoluble, two-dimensional, non-thermosetting and cationic. They contain at least 5 to 10 (and if desired more than amino nitrogen atoms per macromolecule and have molecular weights between about SOD-1,000 and 103000, and when present in adsorbed state in paper confer dry strength without conferring wet strenlgth.

POLYMER A The following illustrates the preparation of a watersoluble non-thermosetting cationic two-dimensional polyamine suitable for the manufacture of paper according to the present invention.

Into a l-liter, three-necked flask fitted with a stirrer, condenser, thermometer, and dropping .fun-nel are placed 93 g. (1.2 mols) of aqueous (commercial) methyl- .amine, 12 g. (0.2 mol) of ethylenediamine, and ml. of water. 148 g. (1.6 mols) of epichlorohydrin is added dropwise over minutes with cooling at 4347 C.

The solution is heated for two hours at 63-67" C., after which 24 g. of sodium hydroxide in ml. of water is added dropwise over 90-120 minutes with the solution at that temperature.

The solution is maintained at 55-65 C. until its viscosity at that temperature is equal to the viscosity of the Gardner-Holdt L standard liquid at that temperature. At this point the epichlorohydrin has substantially completely reacted (as shown by an analysis for ionic chlorine) and the polymer is short of its gel point. The resin solution is neutralized with glacial acetic acid to a pH of 4.5 and is cooled to room temperature. The solids content of the solution (determined by evaporating -a sample to dryness at C.) is 45% by weight. The polymer has an estimated average molecular weight of 2,000 and contains about 12 amino nitrogen atoms per macromolecule.

The polymer solution is stable for many months at room temperature.

In this example, the ratio of the total functionality of the epichlorohydrin (which is bifunctional to the methylamine (which is bifunctional) and the ethylenediamine (which is tetrafunctional) is or 1: 1. The two amines and the epichlorohydrin are thus reacted in functional equivalence. The molar ratio of the methylamine to the ethylenediamine is 6:1.

The polymer has the theoretical structure:

I CH2 POLYAMINE B The following illustrates the preparation of another polymer suitable for use in the present invention.

To 31 g. (1 mol) of methylamine in 40% aqueous solution is added 3.4 g. (0.2 mol) of ammonia as a 28% solution. Epich'lorohydrin (120 g., 1.3 mol) is added dropwise over 90 minutes While the reaction mixture is maintained at 4045 C. The reaction mixture is then heated at 65 C. for 2 hours. Sodium hydroxide solution (26 g. in 75 ml. of water) is then added and the resulting solution is heated at 60 C. until its Gardner-Holdt viscosity is T (about 2 hours). The product is then cooled and acidified to pH 4.5 with concentrated HCl. The solution has a solids content of 50% and is stable for at least several months at 25 C.

In this example, the total functionality of the epichlorohydrin to the methylamine and to the ammonia is or 1:1. The amines and the epichlorohydrin are thus reacted in functional equivalence. The molar ratio of the methylamine to the ammonia is 5:1.

The polymer has the theoretical structure:

CHOH

On the average, the linear chains are cross-linked at every sixth nitrogen atom.

The polymer has an estimated molecular weight of 1,500 and contains about 18 amino nitrogen atoms per macromolecule.

POLYAMINE C The following illustrates the preparation of a nonthermosetting cationic two-dimensional polyamine containing amide groups as spacers which can be used for the manufacture of paper 01f the present invention.

A water-soluble cationic substantially linear polyamidepolyamine [substantially composed of units] and having a viscosity in substantially anhydrous state at 150 of 1,200 centi-p'oises [prepared by heating 237 g. (1.25 mols) of tetraethylenep'entamine 'with 183 g. (1.25 mols) of adipic acid at 150 C. for about 2 hours] is mixed with suflicient water to form a 35% solution by weight and is stirred at 90 C. until dissolved. The viscosity of the solution at C. is less than 10 centipoises. The polyarnidepolyamine contains three secondaryamine groups per pair of amide groups present therein, and has a molecular weight of about 2,200. The polymer is consequently composed of about 7 of the units shown above. The groups of three amino nitrogen atoms are separated by 12 ionically inert atoms.

There is then slowly added with stirring 21 g. (0.23 mol) of epichlorohydrin (about of the total amount to be added). When this has reacted (after about'90 minutes), 4.75 g. of additional epichlorohydrin is added very slowly. After this increment has reacted, the reaction mixture at 35% polymer solids and at 80 C. has a viscosity of 100 centipoises. The total amount of epichlorohydrin added is 0.275 mol, equivalent to 0.09 mol of epichlorohydrin per amine nitrogen atom of the polyamidepolyamine. The solution is acidified to pH 4.4 by addition of concentrated hydrochloric acid and is cooled to room temperature.

Based on viscosity measurements the molecular weight of the resulting polymer is 6,700.

POLYAMINE D This polymer is prepared by the method employed for the preparation of polymer C except that 190 g. (0.275 mol) of methylenebisacrylamide is used as cross-linking agent in place of the epichlorohydrin used for the preparation of polymer C. A similar polymer is obtained.

POLYAMINE E This polymer is prepared by the method employed for the preparation of polymer C except that 27.3 g. (0.275 mol) of 1,2-dichloroethane is used as cross-linking agent in place of the epichlorohydrin used for the preparation of polymer C. A similar polymer is obtained.

POLYAMINE F The following is a polyamine which can be employed to make paper according to the present invention.

To one mol (189 g.) of tetraethylenepentamine at 1 C. is added 50 m1. of water and, dropwise with The following illustrates the dry and wet strengths of papers composed of fibers bonded together by an ionically reacted content of the above-described polyamines.

An aqueous suspension of unbleached kr-aft pulp is beaten to a Canadian standard freeness of 500 ml., and adjusted to a consistency of 0.6% and a pH of 6. Aliquots are removed. One is reserved as control, and to this nothing is added. To each of the other aliquots is added 0.5% based on the dry weight of the fibers, of one of the polymers described above. The aliquots are stirred for a few minutes and are then sheeted to form handsheets having a dry basis weight of about lbs. per 25" x 40"/ 500 ream. The handsheets are dried for 1.4 minutes in rotary drum drier at 1|15 C., and in a few instances their dry strengths are determined. Results are as follows:

Dry Burst Tensile Strength 2 Strength,

lb./in. No. Description M.W.1

Lb. Percent Dry Wet Iner.

OHSNH'QifiIIC'QiI 000' cHgNHg-NHg-epl 1, 500 TEPA-adipic-epi 6, 700 TEPA--adipic-MBA o. TEPA-adipic-CIC ILC do Tuna-010 11.01 2,000

1 Approximate. 2 Lb./in. by Mullen instrument, adjusted to 100 lb. basis weight paper. 3 Over control. 4 At 100 lb. basis weight. 5 All handsheets defibered when slushcd in laboratory Hollander containing neutr all water. 6 Tetraethylcnepentaminc. 7 Mcthylenebisacrylamide.

Example 2 Results are as follows:

The following illustrates the dry and wet strength possessed by paper of the present invention as a function P t Dry Burst Strength ercen of the amount of two-dimensional, non-thermosett mg, low Run Polymer Pulp pH molecular weight polymer present. The followmg also Added Lb. Percent illustrates the pulping of this paper and the reprocessing of the resulting pulp into paper having the same proper- 1 N r ties as the original paper. 2,2 it 0 A stock of southern unbleached kraft pulp is beaten to 0. 3 6.0 96.0 25.5) a freeness of 600 ml. (Canadian standard) diluted to a 8:2 1? 22:8 13:3 consistency of 0.6%, and adjusted to pH 6. Aliquots are taken. One aliquot is reserved as control, and to this 1Basedondryweightofthefibers. iK

nothing is added. To each of the others is added polymer C in the amounts respectively shown in the table below, and the pH is again adjusted to 6.

The aliquots are sheeted to form handsheets having a basis weight of about 100 lb. (25" X 40/500) and the handsheets are dried for 1.5 minutes at 240 F. The Mullen dry strength values of the sheets are determined. Results are as-follows:

Dry Burst Strength Percent Run No. Polymer Wet Added 1 Lb. Percent Strength Incrfi None 74. 5 0. 2 84. 5 13. 4 0. 3 92.0 24. 2 0. 5 108. 0 45. 0

- 1 Based on dry weight of the fibers.

2 Lb./in. by Mullen instrument, adjusted to 100 lb. basis weight paper. 3 Over control. 4 Defibers when slushed in laboratory Hollander containing neutral water at 20 0.

Example 3 2 Mullen, adjusted to lb. basis weight. 3 Over control.

These results indicate that best dry strength with this polymer is obtained when the paper has a pH of 6.0.

' Example 4 The following illustrates the effect of alum on the dry strength imparted by the polymer.

The procedure of Example 2 is repeated, except that the alum is added in advance of the polymer in amount shown in the table below, and the pH of the pulp is adjusted to pH 6 before addition of the polymer. Results are as follows:

Percent Percent Dry Burst Percent Run No. Alum 1 Polymer 1 Strength, lb. Change None None 77.0 Control 0.5 None 73. 5 -4. 5 None 0. 5 103. 0 +23. 8 0. 1 0. 5 97. 5 +26. 5 0. 2 0. 5 98. 5 +27. 5 0. 3 0.5 101. 5 +38. 0 0. 4 0. 5 98. 5 +27. 5 1.0 0.5 93. 0 +21. 0

1 Based on dry weigut of the fibers.

The data show that while alum is tolerated well, .best dry strength is obtained in the absence of this material.

0 We claim:

1. Paper of improved dry tensile strength but of low wet strength substantially composed of unbleached waterlaid cellulose fibers bonded together by an ionically adsorbed content of a normally water-soluble, two-dimensional, non-thermosetting cationic polyaminencontaining 9 at least 10 amino nitrogen atoms per macromolecule and having a molecular weight between about 1,000 and 10,000.

2. Paper according to claim 1 wherein the polyamine contains more than 25 amino nitrogen atoms per macromolecule.

3. Paper according to claim 1 wherein the amino nitrogen atoms of said polyamine are separated by 10 to 20 ionically inert atoms.

4. Paper according to claim 1 wherein the fibers contain more than 1% by weight of acidic lignin material.

5. Paper according to claim 4 wherein the polyamine is present in sufficient amount to neutralize substantially all of said lignin material.

6. Paper according to claim 1 wherein the polyamine is a methylamine-ethy1enediamine-epichlorohydrin polymer.

7. Paper according to claim 1 wherein the polyamine is a methylamine-ammonia-epichlorohydrin polymer.

8. Paper according to claim 1 wherein the polyamine is a tetraethylenepentamine-adipic acid-epichlorohydrin polymer.

9. Paper according to claim 1 wherein the polyamine is a tetraethylenepentamine-adipic acid-1,2-dichloroethane copolymer.

10. Paper according to claim 1 wherein the polyamine is a tetraethylenepentamine-1,2-dichloroethane polymer.

References Cited by the Examiner UNITED STATES PATENTS 2,834,675 5/1958 Jen et al 162164 2,884,057 4/1959 Wilson et a1 162164 3,019,166 1/1962 Lundberg et a1 162-168 3,086,961 4/1963 House et a1. 162l64 DONALL H. SYLVESTER, Primary Examiner.

S. LEON BASHORE, Examiner. 

1. PAPER OF IMPROVED DRY TENSILE STRENGHT BUT OF LOW WET STRENGTH SUBSTANTIALLY COMPOSED OF UNBLEACHED WATERLAID CELLULOSE FIBERS BONDED TOGETHER BY AN IONICALLY ADSORBED CONTENT OF A NORMALLY WATER-SOLUBLE, TWO-DIMENSIONAL, NON-THERMOSETTING CATIONIC POLYAMINE CONTAINING AT LEAST 10 AMINO NITROGEN ATOMS PER MACROMOLECULE AND HAVING A MOLECULAR WEIGHT BETWEEN ABOUT 1,000 AND 10,000. 